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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation Roll Forward Analysis Roll Forward    
Beginning balance $ 1,384 $ 1,180
Additions and revisions of prior estimates 7,084 399
Reductions to liabilities due to revision of estimates   (211)
Accretion expenses 514 16
Ending balance $ 8,982 $ 1,384